The invention relates to flexible polyurethane foams that have been produced from polyisocyanates and polyether polyols in the presence of double metal cyanide (DMC) catalysts and that have at least one ethylene oxide-propylene oxide mixed block, as well as a process for their production.
The expression flexible polyurethane foams denotes foams that exert a small resistance to a pressure stress and that are open-cell, air-permeable and that can be reversibly deformed. The properties of flexible polyurethane foams substantially depend on the structure of the polyether polyols, polyisocyanates and additives such as catalysts and stabilisers that are used in their production. As regards the polyethers, the functionality, chain length as well as the epoxides used and the resultant reactivity of the hydroxyl groups have the greatest influence on the characteristics of the foam.
The production of polyether polyols is mostly carried out by base-catalysed polyaddition of alkylene oxides to polyfunctional starter compounds such as for example alcohols, acids, amines, etc. (see for example Gum, Riese & Ulrich (Editors): “Reaction Polymers”, Hanser Verlag, Munich 1992, pp. 75-96). After completion of the polyaddition the catalyst is removed from the polyether polyol in a very complicated process, for example by neutralisation, distillation and filtration. The long-chain polyethers have to be freed particularly carefully from catalyst residues since otherwise undesirable secondary reactions such as for example the formation of polyisocyanurates, may take place during the foaming. The residual content of potassium and sodium ions in the polyether polyol amounts to only a few ppm. Only polyether polyols with a very low alkali metal content are suitable for the production of polyurethane elastomers and flexible polyurethane foams. The polyether polyols produced by base catalysis also have the disadvantage that with increasing chain length, the content of monofunctional polyethers (so-called monools) constantly increases and the functionality decreases.
In order to circumvent the aforementioned disadvantage, it is recommended in the field of polyurethane elastomers to employ polyether polyols that are produced by using double metal cyanide (DMC) catalysts, and that accordingly have very low contents of allyl ethers (monools) and thus exhibit a higher functionality. Such production processes have been known since the 1960s (U.S. Pat. No. 3,427,256, U.S. Pat. No. 3,427,334, U.S. Pat. No. 3,427,335). The disadvantage of this production method is however the very complicated and expensive removal of the catalysts.
In more recent patent applications (for example EP-A 700 949, EP-A 761 708, WO 97/40086, WO 98/16310, DE-A 19 745 120, DE-A 19 757 574, DE-A 198 102 269) highly active improved DMC catalysts are described, which on account of their very high activity can be used in such small amounts (catalyst concentration≦50 ppm) that a separation of the catalyst from the polyether polyol is no longer necessary. In this way a more economic production of the polyether polyols is possible compared to the conventional base-catalysed process. These products may be used directly for the production of polyurethane elastomers.
The disadvantage however is that conventional, low molecular weight starter compounds such as for example propylene glycol, glycerol and trimethylolpropane cannot in general be alkoxylated with DMC catalysts. The DMC catalysts therefore in general require the use of oligomeric propoxylated starter compounds that are obtained beforehand from the aforementioned low molecular weight starters, for example by conventional alkali catalysis (generally with KOH), followed by complicated working-up, by for example neutralisation, distillation and filtration.
German patent application 198 17 676.7 describes a process for the production of long-chain polyether polyols that is completely free of any working-up stage, in which first of all the pre-propoxylated starter compounds are obtained by catalysis with perfluoroalkyl sulfonates (preferably triflates) of metals of Group III A of the Periodic System of the Elements (corresponding to the 1970 IUPAC Convention), which are then converted without separation of the catalyst and working-up, by means of highly active DMC catalysts into long-chain, high molecular weight polyether polyols. An extremely economical production of long-chain polyether polyols is possible in this way.
The disadvantage however is that poly(oxypropylene)polyols that can be produced very economically by these highly active DMC catalysts without any separation of the DMC catalyst are not suitable for the production of flexible polyurethane foams. The use of such polyether polyols in flexible foam formulations leads to serious crack formation.